1. Field of the Invention
The present application is related to Ziegler-Natta catalysts, and particularly to a method for making a Ziegler-Natta catalyst.
2. Background of the Art
Ziegler-Natta catalysts are advantageously used in olefin polymerization reactions to produce stereoregulated linear polymers. It is known that Ziegler-Natta catalysts may be formed from a silica support material, a magnesium-halide compound, a transition metal compound, electron donor compound(s) and an organo-aluminum cocatalyst. The transition metal is an active catalytic ingredient and the magnesium-halide compound may also be thought of as being active to the extent that it acts as a synergist to increase the overall catalytic productivity of the transition metal. The electron donor compounds and organo-aluminum cocatalyst are important because they enable the Ziegler-Natta catalyst to catalyze the polymerization of isotactic polymers. The silica material is inactive and does not increase polymerization reaction rates.
It is known that silica-based Ziegler-Natta catalysts may be produced from high-porosity silica particles in the diameter range of about 10–100 microns, which may be coated with 1 or more relatively thin layers of a magnesium-halide compound (e.g., MgCl2) to form a catalyst support, which may be treated with a transition metal to form a catalyst. However, this type of catalyst generally comprises a high ratio of inactive silica relative to the active ingredients. Further, the catalyst supports may need to be mechanically treated to produce generally spherical, or spheroidal, catalyst supports of the appropriate diameter and particle diameter distribution, which will form catalyst particles of the appropriate diameter and particle diameter distribution and, thus, will facilitate efficient polymerization reactions.
U.S. Pat. Nos. 4,293,673 and 4,376,062, both to Hamer et al., disclose methods for forming silica based catalysts with very small silica particles, no larger than about 0.05 micron, which are mixed in a slurry to form solid agglomerations comprising a number of silica particles mixed with active ingredients. While Hamer utilizes silica particles which are very small, the resulting catalysts still comprise at least 10 wt % inert silica material. This is undesirable because the higher the relative amount of inert silica material the lower the overall activity of the catalyst, and the higher the amount of catalyst residues in a polymer formed therewith. Thus, the production costs of the polymer, per weight unit of catalyst, is higher for catalysts containing relatively large amounts of inert material. Further, higher amounts of catalyst residues may detrimentally affect polymer processing and/or the physical properties of products made with the polymer.
Significant factors for controlling the polymerization reaction include the physical features of the catalyst particles, i.e., particle shape, size, and particle size distribution. The magnesium halide (e.g., magnesium chloride, MgCl2) serves as a support for the transition metal catalyst. The particles of magnesium halide are formed into particles preferably by using a porous inorganic oxide (e.g. silica, alumina, etc.) as a nucleation aid. The magnesium halide can be deposited on a supporting core provided by the porous inorganic oxide particle. It is desirable to control the nucleation of the magnesium halide to provide consistent results.
A variety of technique exist for forming magnesium halide into a particular shape and size for use as a polymerization catalyst support. However, many of these methods are either elaborate, expensive, or have limited success in terms of optimizing both the shape and particle size distribution of MgCl2. It would be advantageous to have a simple and relatively inexpensive method for producing MgCl2 with a specified particle shape, size, and particle size distribution.